The present invention relates to a method and device for measuring gas permeability through a porous membrane-like material, in which method the volume flow rate through a sample is measured at a certain pressure. In addition to paper, the material may be various kinds of wires and felts. Measurements are performed by placing a sample against a chamber, after which a flow connection is made from the chamber to a source of constant pressure. The predetermined pressure-difference created by it is used to create a stable gas flow through the sample and the so-called volume flow rate is measured, this being proportional to the permeability of the material. The invention relates in particular to the investigation of membrane-like materials, such as greaseproof papers, which are only poorly permeable by gas.
Methods and devices for measuring the gas permeability of a paper are known from patent publications U.S. Pat. No. 4,191,046, U.S. Pat. No. 4,462,248, UK 2 018 436, and UK 2 038 002. In these, the air permeability of a paper is measured by determining the average air flow permeating through a sample of a known surface area at a certain pressure difference. The known devices according to the patent publications are perfectly applicable to the measurement of air-permeable papers. In such devices, the volume flow rate is measured using either thermic flow meters, or with the aid of the pressure difference measured over a throttle.
All of the aforementioned patent publications are actually particularly intended for measuring the air permeability of cigarette papers. However, the reliable measurement of the grease-proofing of baking papers, for example, is not possible using the known devices referred to above, or else it is extremely difficult. The paper industry uses the so-called Linden method and measuring device to determine the air permeability of various paper materials. In this method, a standard quantity of compressed air is blown through a sample piece as a function of time. A corresponding method is also disclosed in publication U.S. Pat. No. 4,385,517. Particular problems in the measurement of poorly air-permeable materials, such as the greaseproof baking papers referred to above, are the poor repeatability of the results, and the slowness of the measurement. In addition, paper materials live with pressure, which makes it difficult to perform a repeatable measurement.
Devices according to the aforementioned patent publications give a stable result after a long waiting period, if even then. For example, a flow connection according to publication U.S. Pat. No. 4,191,046, used with a more sensitive flow sensor, behaves with an airtight sample in such a way that the volume flow rate displayed by the flow meter fluctuates over a considerable range. The problem does not appear when the volume flow rates are large, such as when measuring cigarette papers, which is, after all, the intended purpose of the device according to the publication. Measurement of the pressure difference over a throttle, disclosed in the other publications, is not suitable for low volume flow rates.
German patent publication DE 1 063 832, discloses an apparatus for measuring air permeability. In this device, there are three measurement zones, the smallest of which permits the measurement of the air permeability of even greaseproof papers. In the second and third measurement zones (II and III), a throttle (K2 or K3) is used to directly determine the volume flow rate created by a pressure difference. In the first measurement zone (I), a corresponding throttle is connected in the flow circuit after the sample and the pressure difference over the sample is measured, because the pressure difference over the throttle would be too small. In this case, the volume flow rate cannot be measured precisely. The arrangement is quite complicated and requires a sample-holder that is absolutely airtight on both sides of the sample.
It is wished to keep the surface area of the sample to a reasonable size, to be able to observe possible local variations in a sheet of paper.
The present invention is intended to create a new kind of method and device for measuring gas permeability through a porous membrane-like material, in particular dense paper. The invention is intended to avoid the problems of the state of the art. The characteristic features of the method according to the invention are disclosed in the accompanying claim 1. The characteristic features of a corresponding device are disclosed in the accompanying claim 8. The invention is largely based on the observation that the flow through the sample will stabilize very rapidly, if there is a throttle, preferably a long, narrow tube, in the flow duct, immediately before the sample. Even a small restriction has been shown to be advantageous. Other applications and advantages of the invention will appear in connection with the later embodiments.
In the following, the invention is described by reference to the accompanying figures, which show one measurement device according to the invention and its details.